The present invention relates to a device for reading from and/or writing to optical recording media in which both zeroth-order and first-order diffraction beams fall onto a photodetector.
Devices of this type have the disadvantage that in some instances complex diffraction patterns are produced on the photodetector on account of the superposition of the zeroth- and plus/minus first-order diffraction beams, which diffraction patterns have undesirable interfering influences on signals derived from the output signals of the photodetector even in the event of slight displacement of the beam incident on the photodetector relative to the optical axis.
The object of the invention is to obtain a reduction in the interfering influences in a signal derived from photodetector signals, a signal such as, for example, a focus error signal, in particular one obtained by the astigmatism method, or a track error signal. In this case, the interfering influences can be caused, inter alia, by the superposition of diffraction beams of the zeroth and first or higher diffraction order, by the displacement of the light spot on the photodetector as a consequence of desired or undesired displacement of one or more optical elements relative to the optical axis, or by a combination of these or further interfering influences, for example ones governed by the design.
This object is achieved by means of the measures specified in independent claims.
According to the invention, a beam splitting means is provided, which splits the light spot falling onto the photodetector into two separate partial spots. In this case, this beam splitting means is advantageously arranged in the beam path upstream of the photodetector and generates two partial beams which engender mutually separate light spots on the photodetector. This has the advantage that the partial spots in each case illuminate areas of identical size in different detector zones even in the event of relative displacement with respect to the optical axis. Situated between the partial spots is an unilluminated boundary zone, which advantageously coincides with a boundary between two detector zones, with the result that although the boundary zone is displaced in the event of displacement of the light spot essentially perpendicularly with respect to the said boundary, the boundary nonetheless remains in the unilluminated boundary zone. The detector zones separated by the boundary thus receive an unchanged intensity component even in the event of displacement of the light spot; an interference component in the derived signal is not event brought about in the first place. The optical recording medium is generally in the form of a disc and provided with an information-carrying layer. This layer usually has concentrically or spirally arranged information tracks having a predetermined distance from one another and a predetermined depth. Arranged on the information track are information items in the form of elongate elements of greater or lesser length, which are also called spots or pits, and which may be depressions, elevations and be reflective to a greater or lesser extent or have optically different properties in another suitable manner. The scanning beam generation means generally has a laser diode and corresponding optical elements. The focusing means serves to focus the scanning beam on an information-carrying layer of the optical recording medium. It is often designed, moreover, in such a way that a radial movement, that is to say in a perpendicular direction with respect to the information track, is simultaneously possible for the purpose of tracking the scanning beam on the information track.
According to the invention, the detector zones are separated by a boundary line running in a corresponding manner to the track direction of the recording medium. This has the advantage that a tracking signal can be derived in accordance with the so-called push-pull method, which signal is largely free from interference components that may be caused by a displacement of the light spot relative to the optical axis. In addition to the tracking method mentioned, it is also possible advantageously to employ any other tracking method in which the output signals of the detector zones, which are separated in accordance with the track direction, are combined and evaluated as track error signal. In the case of the method mentioned, the difference zero corresponds to optimal tracking, and a value of greater than or less than zero corresponds to a deviation from the track to the left or right.
According to a further advantageous refinement of the present invention, the device has a photodetector comprising at least four detector elements, and an astigmatism generation means. The astigmatism generation means serves to generate astigmatism in the beam falling onto the photodetector, which enables a focus error signal to be generated. With optimal focusing of the scanning beam onto the recording medium, a circular light spot is produced on the photodetector. In the event of defocusing, the light spot assumes an elliptical form. Therefore, the photodetector has four detector elements generally arranged as four quadrants, the output signals from diagonally opposite detector elements being combined and the difference between the diagonal sums being used as the focus error signal. The astigmatism generation means is a cylindrical lens, for example, but in this case it is also possible, however, to use any other element which generates corresponding astigmatism in the beam falling onto the photodetector. One advantage of this refinement is that a focus error signal is obtained in accordance with the astigmatism method, which signal is largely free from interfering influences caused by movement of the light spot relative to the photodetector. Such movements of the light spot can generally be caused by undesired displacement of one or more optical elements from the respectively optimal position. This is governed for example by ageing, temperature expansion or by sub-optimal adjustment or the like.
According to the invention, the beam splitting means has a light-influencing strip. This has the advantage that it becomes possible to separate the beam into two partial beams in a simple manner, the boundary zone of the photodetector not being covered by any of the partial beams. The light-influencing strip is advantageously an opaque strip; the boundary zone of the photodetector is thus shaded. The opaque strip is arranged in the beam path from the recording medium to the photodetector, to be precise parallel or perpendicular to the track direction and, at the same time, parallel to a boundary between two detector zones of the photodetector, a so-called xe2x80x9cdark linexe2x80x9d.
However, the light-influencing strip that is provided may likewise be a light-deflecting strip. An advantageous variant consists in designing the strip as a prism. That component of the light beam which falls onto the prism is deflected onto a zone situated outside the photodetector, as a result of which it is likewise possible to obtain shading of the boundary zone. Such a light-deflecting strip is inexpensive to produce.
A further advantageous variant of a light-influencing strip consists in arranging polarization-influencing elements adjoining the strip, while the strip itself has no influence on the polarization of the light passing through it. The light whose polarization is uninfluenced can then be filtered out by means of an analyser. Polarization-influencing elements are, for example, quarter- or half-wave plates which convert linearly polarized light into circularly polarized light or rotates the polarization direction. The analyser used is, for example, a polarization filter, a polarizing beam splitter or another suitable optical element.
It is advantageous for the beam splitting means, in this case the light-influencing strip, in particular, to be situated between a beam splitter and the astigmatism-generating element. This has the advantage that the beam falling onto the recording medium is uninfluenced by the beam splitting means and even that part of the beam path which lies downstream of the astigmatism generation means is uninfluenced by the beam splitting means. The beam splitter serves to deflect the reflected beam coming from the recording medium in the direction of the photodetector, which subsequently falls onto the astigmatism generation means. Consequently, only the returning beam is situated between the beam splitter and astigmatism generation means. The said returning beam is advantageously split by the beam splitting means before it falls onto the astigmatism generation means. Arranging the beam splitting means after the passing-through of the astigmatism generation means could have an interfering influence on the wavefront of the astigmatic beam and hence a negative influence on the determination of the focus error signal.
According to a further aspect of the invention, the beam splitting means has a double prism. The double prism advantageously comprises two identical prisms which have a small angle and are joined together e.g. by cementing. The prisms are advantageously produced in one piece, for example by means of a casting moulding process from molten material or by means of induced polymerization. The advantage of a double prism is that the pencil of rays is divided into two equal halves, that is to say no shaded zone occurs, and, consequently, the total intensity of the beam coming from the recording medium falls onto the detector zones.
The double prism can either form the beam splitting means by itself or additionally be combined with one or more other optical elements. The latter case has the advantage that the combination reduces interfering effects and/or intensifies desired effects.
According to a further refinement of the invention, the beam splitting means has a polarizing screen. The screen preferably has a polarizing strip. This has the advantage that the polarization property of the laser beam whereby the laser beam is linearly polarized is utilized. Therefore, the polarizing screen can also be arranged in that zone of the beam path which lies upstream of the reflection of the scanning beam at the recording medium, without this part of the beam being adversely affected. The beam splitting means is thus arranged nearer to the recording medium or to the focusing means, which enables more accurate positioning with respect to the optical axis and consequently results in better interference signal suppression properties. It is advantageous for a quarter-wave plate to be arranged downstream of the polarizing screen, which plate ensures that the linearly polarized light incident on it has a polarization direction rotated through 90xc2x0 after passing through it on the outward and return paths, the said light, which was allowed to pass through the polarizing screen on the outward path without being influenced, consequently being screened out by the said polarizing screen on the return path in accordance with the configuration of the said screen.
The beam splitting means is advantageously formed by two plane-parallel plates arranged at an angle with respect to one another. This has the advantage that the two partial beams run in a quasi parallel-displaced manner spaced apart from one another. The total light energy thus falls onto the detector; no shading loss occurs. The plates are arranged such that they are tilted at a relatively acute angle with respect to one another. A corresponding effect can likewise be obtained by means of prisms arranged such that they are tilted with respect to one another.
According to a further variant of the invention, the beam splitting means is a double grating element. In this case, two optical gratings having different grating parameters are arranged parallel to one another and perpendicularly to the boundary line of the photodetector. The gratings are designed in such a way that the first-order diffraction beams have a significantly greater intensity than the zeroth-order diffraction beams. As a result of the different grating parameters, the first-order diffraction beams of different gratings have different diffraction angles, as a result of which splitting into partial beams is achieved. Optical gratings have the advantage that they can be produced inexpensively and with high precision.
According to the invention, the beam splitting means has a half-wave plate and a Wollaston prism. The said half-wave plate serves to rotate the polarization direction of part of the incident light beam, while the polarization direction of the other part remains unchanged. It is likewise possible here to combine two half-wave plates in a suitable manner. The Wollaston prism is oriented in such a way that one partial beam leaves the Wollaston prism as the ordinary ray and the other partial beam leaves the Wollaston prism as the extraordinary ray, the said partial beams leaving the said Wollaston prism at an angle with respect to one another. This arrangement also has the advantage that it can be produced inexpensively.
An advantageous development of the invention provides for a further photodetector to be provided for the purpose of detecting further partial spots. This has the advantage that screened-out components of the light beam which lie outside the actual photodetector can be utilized to form further signals, for example to form an HF or data signal. The total available intensity which can be evaluated is thus optimally utilized.
The invention provides for the beam splitting means to be arranged such that it is adjustable in the beam path. This has the advantage that it can be adapted to changed ambient conditions, such as, for example, temperature influence, ageing, displacement of the optical components relative to one another, for example due to the influence of an impact or the like. In this case, the adjustment can be carried out at intervals, if appropriate by means of manual or partly automated adjustment intervention, or else in automated fashion at intervals of greater or lesser duration. Consequently, further improved interference signal suppression is obtained; adjustment of the beam splitting means can be carried out more easily than the readjustment of what may be a plurality of optical components with respect to one another. The adjustable beam splitting means is advantageously an electrically drivable element which, if appropriate, also manages without mechanically moving parts. An appropriate example here is a liquid-crystal element having a plurality of strip elements which can be darkened independently of one another and have an appropriately small dimension. This enables adjustment both of the position and of the width of the strip that can be darkened. Self-adjustment of the beam splitting means is also possible, for example by means of coupling to a component which influences the displacement of the light spot with respect to the optical axis. This makes it possible to effect a compulsory displacement of the position of the beam splitting means with respect to the optical axis. It is advantageous for the coupling to be such that the symmetry of the partial light spots on the detector zones is as great as possible in every pertinent operating state.
A further refinement of the present invention provides for the beam splitting means to be a beam splitting means which splits the light spot into a plurality of partial spots. In this case, two, four, six or else a higher number of partial light spots are generated. An odd number of partial light spots also lies within the scope of the invention. The splitting into a plurality of partial light spots has the advantage that the boundary zones situated between the partial spots cover a plurality or else all of the boundaries between neighbouring detector zones or detector elements, with the result that in the event of a displacement of the light spot in any direction, a displacement of the intensity distribution on the individual detector zones or detector elements, the said displacement affecting the individual detector signals, does not occur since the partial spots in each case remain completely on the detector partial-zone assigned to them. Any reduction that may be caused by the beam splitting means in the respective intensity of the light falling onto the photodetector is negligible in comparison with the advantage of the reduction in interference components in signals derived from the photodetector.
According to the invention, the beam splitting means is coupled to another optical element of the device. This has the advantage of simple production without any additional outlay in terms of assembly and adjustment, and also of integration in existing designs without difficulty. For example, a strip of the beam splitting means is in this case arranged on the surface of a lens.
The invention further provides for the double prism to have at least one double-refracting prism. This has the advantage that the splitting is possible by way of the polarization of the light. The plane of polarization is rotated for example by a half-wave plate which is passed through twice.
The method claim specifies an advantageous method for a device according to the invention. In this case, too, advantageous developments present themselves, in a similar manner to that specified for the apparatus claims.
Further advantages of the invention also emerge from the following description of the exemplary embodiments. In this case, this description serves to elucidate the invention, which is not restricted to the exemplary embodiments.